Graphene Nanocomposites

Article pubs.acs.org/IECR

High Activity Ti3+-Modified Brookite TiO2/Graphene Nanocomposites with Specific Facets Exposed for Water Splitting Qianqian Shang,†,∥ Xiang Huang,‡,∥ Xin Tan,‡ and Tao Yu*,†,§ †

School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300072, China School of Science, Tibet University, Tibet, 850000, China § Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China ‡

S Supporting Information *

ABSTRACT: Brookite TiO2 exhibits promising photocatalytic activity in photoreduction; however, it is least known due to poor stability. We have synthesized Ti3+-modified brookite TiO2/graphene nanocomposites with specific facets exposed successfully. They show highly improved photoreduction activity for water splitting into H2 with excellent stability. The well-crystallized brookite TiO2 nanorods are surrounded by four reductive (211) facets with high conduction band potential and growth along the [001] direction, indicating that they have high photoreduction ability because more reductive electrons will be excited. By combining spectroscopic techniques and electrochemical analysis methods, the outstanding activity could be linked with the synergistic effects of highly exposed (211) facets, Ti3+ defects, and graphene. Their formation mechanism and the effects in the enhanced photoreduction activity have been discussed in detail. In addition to promoting the separation of photogenerated e−− h+ pairs effectively, the midgap state was introduced and the absorption ability was improved.

1. INTRODUCTION With the growing depletion of traditional fossil fuels, water splitting into H2 over photocatalysts becomes a more important worldwide issue.1−5 Due to the economic efficiency and high chemical stability, titanium dioxide (TiO2) as an UV-lightdriven photocatalyst has been extensively applied in solardriven clean environmental and energy technologies.5−9 Compared with the widely investigated anatase and rutile, the modification and preparation of pure phase brookite with enhanced photocatalytic activity is rarely reported due to the technical difficulty.10−13 However, the chemical stability of brookite is higher than that of anatase in nature, and the conductive band potential of brookite is more negative than those of the other two crystal structures; thus it exhibits promising photocatalytic activity in photoreduction.13−16 In recent decades, many researchers have dedicated themselves to improving the photocatalytic performance of brookite by morphology regulation.11−18 However, their photocatalytic activities are shape-dependent, and few studies have shown brookite with excellent photocatalytic activity up to now.15,27 It is generally known that the performance of brookite © XXXX American Chemical Society

TiO2 was limited by the recombination of photogenerated carriers, the wide band gap (3.14 eV), and the inefficient absorption of pollutants.19−21 As a two-dimensional (2D) carbon material, graphene (Gn) has been widely studied in the field of composite materials. To take advantage of its excellent electroconductivity and large specific surface area, combining TiO2 with Gn is effective in extending the response range toward light, improving its adsorbent capacity, and increasing the mobility of charge carriers.22−25 Although clear recognition about synthesizing Gn−TiO2 nanocomposites has been obtained, very few concerns are focused on the brookite phase. In addition, the facet type of brookite TiO2 exposed could affect the charge migration efficiency, surface adsorption ability, and redox potential of charge carriers.26,37,38 Zhao et al. proved that the surface energy of (121) facets is higher than that of (211) facets Received: Revised: Accepted: Published: A

March 30, 2017 July 16, 2017 July 18, 2017 July 18, 2017 DOI: 10.1021/acs.iecr.7b01263 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research by theoretical calculations.13 Lin et al. verified that the activity of brookite has a close relation to the exposed facet, and the coexistence of (210) and (101) facets with different surface energies is effective in separating electrons (e−) from holes (h+).7 Also, the valence band (VB) and conduction band (CB) potentials were greatly influenced by the surface atomic arrangement.13,38 It should be noted that the selective controlling of specific crystal facets, especially high energy facets of brookite TiO2 exposed, seems even harder. Moreover, Ti3+ defects have been widely applied in wide band gap semiconductors such as TiO2 to improve the photocatalytic performance by introducing the midgap and enhancing the electroconductivity.28−33 Qiu et al. have synthesized the highly efficient Ti3+-modified TiO2 for H2 production with high stability.30 However, the Ti3+ defects are not stable enough in air because Ti3+ is easily oxidized into Ti4+ by oxygen.30−33 Therefore, stabilizing the Ti3+ defects on brookite TiO2 is still a challenge. As mentioned above, we have synthesized high-quality Ti3+modified brookite TiO2/Gn nanocomposites with specific facets exposed successfully, which show excellent photocatalytic efficiency for water splitting with high stability. The obtained brookite TiO2 is mainly surrounded by four reductive (211) facets with high CB potential which are grown along the [001] direction. The coexistence of Ti3+ defects and Gn could facilitate the separation of photogenerated e− from h+, broaden the photoresponse wavelength, and increase the absorption capacity of pollutants.

from 280 to 600 nm. The surface composition and valence band spectra were detected by X-ray photoelectron spectroscopy (XPS; S4 Pioneer, Al Kα source). Electron paramagnetic resonance (EPR) spectra were characterized on a Bruker A320 EPR spectrometer. Fourier transform infrared spectra (FTIR; Thermo Scientific, Nicolet 6700) were collected on an IR spectrometer. 2.3. Photocatalytic Performance Evaluation. The photoreduction performances of B, TB, and TGB were measured by splitting water into H2. First, the sample (0.1 g), methanol (20 mL, 99.99% AR, sacrificial reagent), and chloroplatinic acid (133 mL, 0.5 wt % Pt, cocatalyst) were added into 220 mL of deionized water under stirring, respectively. The photodeposition process of Pt was started under full wave band light. After Pt loading and vacuumizing, H2 evolution was started under the irritation of a 300 W Xe lamp (HSX-F300, λ = 365 nm, 100 mW/cm2, Beijing NBet Technology Co., Ltd.). During the whole process, Ar (99.99%) was used as the carrier gas. The concentration of produced H2 was detected by an online GC-2014C gas chromatograph apparatus (Shimadzu, Japan). The photocatalytic activities of B, TB, and TGB were further confirmed by decomposing the liquid rhodamine B (RhB; 10 mg/L, 50 mL) in a quartz reactor with an effective volume of 75 mL (Φ = 10 cm) under a magnetic stirrer. A 300 W Xe lamp was used as the light source (HSX-U, λ = 365 nm, 100 mW/ cm2), and the solution was cooled by circulation water; the distance between UV light and RhB liquid level was kept at 10 cm. The dark reaction was performed to achieve the saturation adsorption before photoreaction. Once the photodegradation started, 4 mL of the mixture was taken every 15 min to analyze the concentration of RhB. To investigate the photocatalytic oxidation pathway further, additional tert-butyl alcohol (TBA, 10 mM, 5 mL) as •OH scavenger, AgNO3 (10 mM, 5 mL) as e− scavenger, p-quinone as O2− scavenger. and EDTA−2Na (0.1 g) as hole scavenger were added into the above solution, respectively. 2.4. Photoelectrochemical Characterization. The samples were coated onto FTO conductive glass (MTI Corp., China) with the use of a doctor-blading method.35 A CHI660E electrochemical workstation was used to detect the photoelectrochemical properties of the samples. The prepared TiO2 film (1 × 1 cm2), Pt wire (0.5 × 37 mm), and Ag/AgCl (R0303) as work, counter, and reference electrodes, respectively, were immersed in 1 mol/L Na2SO4 electrolyte, and the above solution was degassed with argon. A 300 W Xe lamp (λ = 365 nm, 100 mW/cm2) was used as the light source, kept at 10 cm with the work electrode. Electrochemical impedance was measured in the frequency range from 1 × 106 to 0.1 Hz and 5 mV of amplitude under open-circuit potential and illumination. Mott−Schottky (M−S) plots were detected at potentials from −1 to 1 V with a frequency of 2 kHz in the dark. The photoresponses (I−t) were recorded under intermittent irradiation every 40 s with five recycles at −0.4 V.

2. EXPERIMENTAL SECTION 2.1. Synthesis. The brookite TiO2, Ti3+-doped brookite TiO2, and Ti3+-doped brookite TiO2/Gn nanocomposites, denoted as B, TB, and TGB hereinafter for convenience, were synthesized with the use of a hydrothermal method. 2.1.1. Synthesis of B. First, 20 mM tetrabutyltitanate (TBOT) was dissolved in 50 mL of deionized water, followed by 4 g of urea and 5 mL of sodium lactate liquor (60%) under ceaseless agitation. After 30 min, 1 M NaOH aqueous solution was added until pH 11. Then the above solution was transferred to a Teflon autoclave with a capacity of 100 mL and kept at 200 °C for 12 h in draft drying cabinet. After it cooled, the product was rinsed with deionized water and ethanol alternately to pH 7.0, and finally dried at 70 °C overnight. 2.1.2. Synthesis of TB. The synthesis was conducted under the same conditions as described in section 2.1.1 except that an additional 0.1 g of NaBH4 was added before the addition of NaOH solution. 2.1.3. Synthesis of TGB. The synthesis was conducted under the same conditions as described in section 2.1.2 except that an additional 20 mg of prepared graphite oxide (GO) was dispersed into 50 mL of deionized water under ultrasound for 60 min initially. 2.2. Characterization. The crystal structure was tested by an X-ray diffractometer (XRD; Bruker D8-Focus, Cu Kα source, λ = 0.1542 nm) and inVia reflex Raman spectroscope (Raman, λ = 532 nm). A transmission electron microscope (TEM; JEM-2100F, Japan) and scanning electron microscopy (SEM, S-4800, Japan) were used to observe the morphology and lattice structure of the samples. A UV-2700 spectrophotometer was used to measure the UV−visible spectra (Shimadzu, Japan). Photoluminescence (PL) was obtained with an X-ray fluorescence spectrometer (Spectro, Germany)

3. RESULTS AND DISCUSSION 3.1. Crystal Structure, Morphology, and (211) Facet Formation Mechanism. In Figure 1, the XRD diffraction peaks of all three samples could be ascribed to the reflection of the orthorhombic single phase brookite TiO2 (JCPDS No. 761934). It is noteworthy that the XRD patterns of TB and TGB are different from that of B by the more pronounced and sharper (211) peak and minor alterations in the relative B

DOI: 10.1021/acs.iecr.7b01263 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 1. X-ray diffraction (XRD) patterns of B, TB, and TGB samples.

intensities and widths of other diffraction peaks. This indicates a preferential lattice elongation in the (211) direction after the addition of NaBH4. No diffraction patterns of carbon species were detected in TGB, which may result from the weak diffraction intensity of graphene. Raman spectra were used to confirm the crystal structure further. In Figure S1, the peaks of various bending modes all correspond to the pure brookite TiO2.13 Additionally, the average particle sizes estimated from the full width at half-maximum (fwhm) of XRD peaks using the Scherrer equation, eq S1, are 48, 76, and 80 nm for B, TB, and TGB, respectively.34 This illustrated that the mean sizes of the samples were enlarged after the addition of NaBH4 and Gn. In order to explore the effects of added NaBH4 and Gn on the morphology of brookite TiO2, TEM and SEM were used to observe the microstructure evolution of B, TB, and TGB samples. Figure 2a displays that B sample consists of a high ratio of squarish nanosheets with smaller size and a low proportion of TiO2 nanorods. In Figure 2b, it is obvious that the ratio of brookite TiO2 nanorods has increased significantly in the TB sample. The dashed line in Figure 2b shows us the unambiguous hexagonal outline of brookite TiO2 nanorods. As shown in Figure 2c, the well-crystallized brookite TiO2 nanorods were distributed on the single thin Gn layer; this illustrates the successful loading of brookite TiO2 nanorods on Gn nanosheet. Figure 2d shows the high-resolution TEM image of the brookite TiO2 nanorods in TB sample. The uniform clear lattices indicate that the sample is highly crystallized, and the lattice distance over a large area is equal to 0.290 nm, which corresponds to the brookite (211) plane. Those above results demonstrate that the addition of NaBH4 is sufficient to prepare brookite TiO2 nanorods with preferred orientation growth of (211) facets, which has been confirmed by the XRD pattern. Based on the fast Fourier transform (FFT; inset of Figure 2d) analysis of lattice fringes, we conclude that the main facets of brookite nanorods consist of (211) and (210) planes, and the (211) facets grow along the [001] direction. The SEM images of B and TB are displayed in Figure 2, parts e and f. The B sample consists of cubic, cuboid nanosheets and sharp-edged nanorods, and the TB sample includes a quite high proportion of brookite TiO2 nanorods, which is consistent with TEM results. The structural illustration of brookite nanorods is shown in the inset of Figure 2f. Combined with the related TEM and FFT images, we speculated that the brookite TiO2 nanorods are surrounded by four equivalent (211) facets and grow along the [001] direction; the two tips are composed of four sharp-edged triangular (210) facets.

Figure 2. Transmission electron microscopy (TEM) images of B (a), TB (b), and TGB (c). High-resolution TEM image of TGB (d). Scanning electron microscopy (SEM) images of B (e) and TB (f) samples; inset shows the structural illustration of brookite nanorods.

The above results demonstrate that the samples are pure brookite and NaBH4 plays an important role in promoting the preferred orientation growth of the (211) crystal plane. As illustrated in Figure 3, when the precursor TBOT was dropped into water, the water-soluble complex [Ti(OH)2(OH2)4]2+ could be formed. Then the layer titanate reacted with Na+ ions and nucleated to HxNa2xTi3O7 particles.11−18 Moreover, with the increase of Na+ ion concentration, more Na+ ions would adhere on the surface of layer titanate that may destroy their intrinsic equilibrium and induce the gradient distributions of charges.10 It has been verified that the (001) plane has the lowest formation energy (0.62 eV) among the (100), (010), (001), (110), (011), and (111) planes of brookite TiO2 under the existence of Na+ ions. Considering the principle of lowest energy, the HxNa2−xTi3O7 structure prefers to grow along the [001] direction as a new layer. Based on the mechanism of Ostwald ripening, the larger particles would grow up into the rod shape at the expense of shrinking the smaller nanoparticles. In addition, it was certified that the surface energy of (211) (0.66 J/m2) facets is lower than that of (210) (0.70 J/m2) facets;11 hence the brookite TiO2 nanorod is exposed with (211) facets. 3.2. Spectral Measurement and Formation Mechanism of Ti3+. Figure 4 shows the UV−visible diffuse reflectance spectra. The maximum absorption wavelengths were measured to be 365, 376, and 384 nm for B, TB, and TGB, respectively. Calculated on the basis of (αhν)1/2 vs photon energy (eV), their corresponding band gap energies are 3.26, 3.2, and 3.12 eV, successively. It is apparent that the C


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Figure 3. Formation mechanism of pure brookite TiO2 with (211) facets exposed (a). Crystal structure of brookite TiO2 growth along the [001] direction (b).

Figure 4. UV−visible diffuse reflectance absorption spectra (a). Plot of (αhν)1/2 vs photon energy (eV) (b). EPR spectra under room temperature (c) and low temperature (d) of B, TB, and TGB samples.

Those can be attributed to the introduction of Ti3+ by added NaBH4. The existence of Ti3+ defects was confirmed by

absorption edges of TB and TGB exhibit red shifts and their corresponding band gaps become narrow compared with B. D


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Figure 5. XPS spectra of Ti 2p (a), O 1s (b), and C 1s (c). FTIR spectra of B, TB, and TGB samples (d).

eV could be attributed to the existence of Gn and adventitious adsorbed carbon, C−OH, and OCOH bonds, respectively.21−25 The OC−OH bonds were formed by the reaction between −OH groups in TiO2 and −COOH groups in Gn or CO2. Compared with B and TB, the increase of the peak at 284.4 V in TGB illustrates that the introduced Gn could improve the absorptive ability of TGB effectively. In addition, it is obvious that much more visible light and near-infrared light are absorbed in the TGB sample (in Figure 4b), which could be ascribed to the added Gn, which is an excellent black carbon material. The chemical bonding modes of B, TB, and TGB were detected by FTIR spectra further. It can be seen that B, TB, and TGB samples have similar profiles of FTIR spectra curves in Figure 5d. The peaks at 400−800 cm−1, 1350 and 2350 cm−1, 1620 cm−1, and 3400 cm−1 were assigned to Ti−O−Ti stretching vibrations, surface adsorbed CO2, CO bond, and O−H groups, respectively.20,39 It should be noted that the increase of the peak at 1620 cm−1 in intensity of TGB implies the successful loading of Gn with more carbon functional groups. The slight increase of O−H groups introduced by the adsorbed H2O and Ti−OH groups on the TiO2 in intensity demonstrated the OV has been produced by added NaBH4, which is helpful in absorbing −OH and H2O.28,31 Based on the above results, it is illustrated that Ti3+ defects can be obtained along with the generation of OV sites by reducing the Ti4+ after the addition of NaBH4, which will produce donor states [OV·Ti3+]+ just below the CB, so the forbidden band of TiO2 would be narrow and thus the light absorption would be improved.29,30 Moreover, Ti3+ as an e− capture agent could inhibit the recombination of e−−h+.31,32 However, only a trace of H2O was introduced in our system, so here the atomic H works as reductant:28

electron paramagnetic resonance (EPR) measurements further. As shown in Figure 4c, the g-value of the peak located at 2.02 corresponds to O2−, and no signal of Ti3+ (g = 1.98) was detected because the surface Ti3+ is easily oxidized to Ti4+ under room temperature.28−32 Thus, we inferred that the paramagnetic Ti3+ defects exist on the surface of the sample. Moreover, it is noteworthy that the intensity of this signal of TB and TGB increased significantly compared with that of B. This indicated that many more Ti3+ defects exist on the surfaces of TB and TGB samples. In Figure 4d, the sharp EPR signals at g = 2.003 of TB and TGB under low temperature could be assigned to oxygen vacancy (OV). The OV always produced a neighbor to Ti3+ defects as a redox couple (Ti4+ + e → Ti3+; O2− − e → O−);33 thus this appeared signal indicates the existence of Ti3+ defects in the bulk of the sample. No peak at g = 2.02 was detected in Figure 4d, which indicates that the surface Ti3+ is hard to oxidize under low temperature. Therefore, the Ti3+ facets exist in both the bulk and surface of brookite TiO2 when NaBH4 is added as the reducing agent. The chemical compositions of B, TB, and TGB were investigated by XPS. In Figure 5a, the two peaks located at 458.3 and 464.1 eV can be assigned to the Ti 2p3/2 and 2p1/2 peaks of Ti4+−O bonds in TiO2, respectively. It is apparent that the two peaks of TB and TGB shift slightly toward lower energy compared with B. It has been verified that the Ti 2p3/2 and 2p1/2 peaks of Ti3+ are situated at 457.6 and 463.5 eV separately.12,15 Therefore, the shift of the Ti 2p peak could be ascribed to the existence of Ti3+ defects introduced by the added NaBH4. In Figure 5b, the two peaks in O 1s XPS spectra of the samples at 529.6 and 531.3 eV indicate the presence of Ti−O bonds and oxygen vacancy (OV), and the intensity of the OV peak was enhanced in the TB and TGB samples. This demonstrates the coexistence of OV neighbor to Ti3+ in the form [OV·Ti3+]+; this introduced midgap could narrow the band gap of brookite TiO2. In Figure 5c, the C 1s XPS spectra of the samples were separated into three peaks. The peaks at 284.4, 286.8, and 288.7

8Ti4 + + NaBH4 + 2H 2O → 8Ti 3 + + NaBO2 + 8H+ E


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Figure 6. Photoluminescence emission (PL) spectra (a), photocurrent density−time (I−t) response curves (b), and Nyquist plots (c) of B, TB and TGB samples.

Figure 7. Mott−Schottky curves (a), XPS valence bands (b), and band structures (c) of B, TB, and TGB samples.

3.3. Electrochemical Detection, Working Mechanism of Ti3+, Gn, and (211) Facets. PL emission spectra are usually used to detect the behavior of photogenerated e−−h+ pairs in semiconductor photocatalyst.36 In Figure 6a, the profiles of emission spectra excited at 360 nm of B, TB, and TGB are similar. The peak at 440 nm can be ascribed to the excitonic photoluminescence, and the small peak at 420 nm is resulted from surface OV defects. It is clearly seen that the intensities of PL spectra of B, TB, and TGB samples show a gradual decrease both at 420 and 440 nm. Because the PL spectra are produced by the recombination of charge carriers, the decrease in intensity indicates that less photogenerated electrons are

recombined with holes. The I−t curves could provide the evidence for the inhibited recombination of photogenerated charge carriers by detecting the transient photocurrent responses. As shown in Figure 6b, the photocurrent increases in the order B, TB, TGB under UV light, so the amounts of recombined photogenerated e−−h+ pairs are in the reverse order. Therefore, the introduced Ti3+ and Gn by adding NaBH4 and GO are effective in inhibiting the recombination of free carriers. Meanwhile, the coexistence of (210) facets and (211) facets with different surface energies could work as oxidation and reduction sites, respectively, to promote the separation of photogenerated carriers.16 F


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Figure 8. Comparison of photocatalytic activities for H2 evolution of B, TB, and TGB sample under UV−visible light (a). Stability test of photocatalytic H2 production activity over TGB sample (b).

H2 occurs in three cycles in 12 h, indicating the high stabilization of TGB. In contrast, Figure S1 shows the photocatalytic activity for degrading RhB. It is noticeable that the B, TB, and TGB samples show different photocatalytic activities for RhB degradation and H2 evolution. This could be attributed to that the high exposure of (211) facet with high CB potential is preferred to the reduction reaction rather than the oxidation degradation reaction. The slight increase in RhB degradation of TGB compared with TB could be ascribed to that the loaded Gn can improve the adsorption of RhB dye and accelerate the transfer of electrons. In Table S2, the surface area of TGB increases significantly due to the addition of Gn. To explore the important roles that plays during the photodegradation process, additional EDTA−2Na (h+ scavenger), tert-butyl alcohol (TBA, • OH radical scavenger), AgNO3 (e− scavenger), and p-quinone (•O2− scavenger) were added. As shown in Figure S2b, in contrast to TB without any scavenger, the activity for photodegrading RhB was almost inhibited after the TBA and AgNO3 were added, respectively, indicating that •OH and e− play a crucial part during the photodegradation process. This corresponds to the above theory, that the e−-related reduction tends to happen. 3.6. Mechanism of Enhanced Photoreduction Performance. In conclusion, the synergistic effect of reductive (211) facets, Ti3+ defects, and Gn contributed to the excellent activity for splitting water into H2 of TGB. Figure 9 shows the

Electrochemical impedance spectroscopy was used to explore the distinct electrical characteristics of the materials and the resistance of charge transfer. Figure 6c presents the Nyquist plots of B, TB, and TGB. It demonstrated that all three Nyquist plots consist of a semicircle and the diameter of the semicircle decreases after the addition of NaBH4 and GO. The big radius at high frequencies corresponds to big charge transfer resistance.38 Thus, we can be speculate that the coexistence of Ti3+, (211) facets exposed, and Gn is indeed effective in improving the electron transfer rate. 3.4. Energy Band Structure. It has been reported that the conduction band potential (Ecb) is very close to the flat band potential (Efb) for n-type semiconductors. The Efb’s of B, TB, and TGB samples were studied by Mott−Schottky (M−S) plots. Figure 7a shows the M−S plot curves as C−2 vs potential. It can be measured that when C−2 = 0, the values of Efb are −0.62, −0.73, and −0.79 V for B, TB, and TGB, respectively. The Efb vs Ag/AgCl can be converted to Efb vs reversible hydrogen electrode (RHE) using the following equation:27 VRHE = VAg/AgCl + 0.059pH + 0.197 (V)

The corresponding values of Efb vs RHE (equal to Ecb) are −0.02, −0.13, and −0.19 V for B, TB, and TGB samples, respectively. Figure 7b shows the valence band (VB) spectra of B, TB, and TGB samples. Their VBs shift slightly from 3.24 to 3.02 to 2.94 eV, respectively. TB and TGB exhibit much lower VB potentials than B (3.24 eV). We can suggest that the lower VB of TB and TGB was caused by the synergistic effect of the introduced Ti3+ defects, Gn, and specific facets exposed induced by the added NaBH4. The band gaps of 3.26, 3.20, and 3.12 eV for B, TB, and TGB have been deduced from the UV−visible spectra displayed in Figure 4. Thus, based on the above results, the band structures of B, TB, and TGB can be speculated, as shown in Figure 7c. It is clearly that TGB shows a higher CB potential than B due to the exposure of reductive (211) facets, which will generate more reductive electrons. 3.5. Photographic Activity Test. The photocatalytic water splitting into H2 was performed to evaluate the photoreduction activity of prepared B, TB, and TGB. In Figure 8a, TGB exhibits the highest H2 productivity among the three samples, followed by TB and B. According to the linear fitting of water splitting into H2, the rate was not decreased with prolonging the time (see the Supporting Information). In addition, the stability of the TGB sample was evaluated by repeating the experiment of H2 evolution process. In Figure 8b, no deactivation of photocatalytic activity of water splitting into

Figure 9. Illustration of photocatalytic reaction mechanism of TGB.

mechanism of enhanced photocatalytic activity of TGB. The preferred orientation growth of (211) facets, Ti3+ defects are caused by the added NaBH4.32 The reductive (211) facets with high CB potential tend to produce more reductive electrons. In addition, the coexistence of (210) facets and (211) facets with different surface energies could drive the separation of charge carriers. The Ti3+ defects can introduce [OV·Ti3+]+ midgap, G


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which could narrow the band gap, thus enhancing the light absorption wavelength. Another potential advantage of Ti3+ and OV is that, as the trapping sites, they could prevent the recombination of photogenerated e−−h+. In addition, Gn as a support could promote the separation of photocarriers and Gn as a carbon material could improve the adsorption capacity.

4. CONCLUSION This study has demonstrated that high-quality Ti3+-modified brookite TiO2/Gn nanocomposites with specific facets exposed were prepared successfully. The added NaBH4 plays a crucial role in the preferred orientation growth of (211) facets by introducing the excessive Na+ and the Ti3+ defects by BH4− ion reducing the Ti4+. Owing to the preferred orientation growth of reductive (211) facets with high CB potential, Ti3+ defects, and Gn, the sample exhibits an outstanding activity for water splitting. Except all three aspects are effective in separating the photegenerated e− from h+, (211) facets with high CB potential could excite more reductive electrons, Ti3+ defects could also broaden the wavelength of light response by introducing midgap states, and Gn could improve the absorptivity of pollutants.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b01263. Scherrer equation; Raman spectra of B, TB, and TGB samples; photocatalytic activity of B, TB, and TGB samples for degrading RhB; linear fitting for water splitting into H2 of B, TB, and TGB samples (y = Ax + B); BET specific surface areas of T, TB, and TGB samples (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-022-2350-2142. ORCID

Tao Yu: 0000-0001-9839-9328 Notes

The authors declare no competing financial interest. ∥ Q.S. and X.H. contributed equally to this work as first authors.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21406164, 21466035), the National Key Basic Research and Development Program of China (973 Program, Nos. 2014CB239300, 2012CB720100), and the Research Fund for the Doctoral Program of Higher Education of China (Nos. 20110032110037, 20130032120019).

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REFERENCES

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